1. Field of the Invention
The present invention relates to an improved cardioplegic solution to protect the heart from ischemia-induced damage incurred during interruption of the blood circulation to the heart during surgery and transplantation.
2. Description of the Prior Art
Surgical procedures for the correction of complex congenital heart abnormalities, placement of cardiac valvular prostheses or repair of defective valves, and bypassing obstructed coronary vessels requires the body to be supported by the heart-lung machine while the heart is rendered quiescent by interruption of its blood supply and briefly perfusing it with a cold solution electrolytes and a high potassium concentration (cardioplegic solution). This enables the surgeon to work in a still and bloodless field to complete the intricate surgical procedures before irreversible ischemic damage is incurred.
The ischemic heart tolerates ischemia for 20-30 minutes before irreversible damage occurs. With the onset of ischemia, the supply of substrates for energy production ceases, and the high energy phosphate adenosine triphosphate (ATP) (which provides energy for contraction and operation of ion pumps in the myocardial cell) is degraded over time to its precursors ADP and AMP. AMP can undergo further degradation at the myocardial membrane to the diffusable purine nucleoside adenosine. Adenosine is also rapidly metabolized to inosine, hypoxanthine and xanthine. With the restoration of blood floow, these nucleosides are washed out of the heart via the circulation. If the ischemia time has been of sufficient length, the level of ATP is reduced, hence less energy for contraction and maintenance of ionic fluxes, and the contractile function of the heart may be diminished or lost. Therefore, methods were developed which would extend the length of time the heart could tolerate ischemia in order to reduce the morbidity and mortality of cardiac operations. Investigations of possible solutions useful in delaying the onset of ischemic damage have involved the use of a multitude of ingredients, but the standard cardioplegic solution in use today contains normal plasma concentrations of electrolytes with the exception of an elevated concentration of potassium which depolarizes the cardiac muscle, rendering it quiescent. The use of hyperkalemic solutions with hypothermia to lower the basal metabolic rate of the cardiac tissue reduces the rate of ATP degradation during ischemia and increases the tolerated ischemic time of the heart during surgery. The protection afforded by these techniques, however, it not optimal in all cases and inadequate myocardial protection during prolonged ischemia is responsible for prolonged weaning from the cardiopulmonary bypass machine, the use of inotropic drugs to support the failing heart postoperatively, and for the mortality associated with postoperative arrhythmias or cardiac failure. Therefore, improvements in the protective cardioplegic solution are needed to reduce the risks attendant with cardiac surgical procedures.